It has already been proposed (see commonly owned German Printed Specification No. 1,929,380) to power a pair of driven shafts, connected with respective endless-track sprockets, from an engine via a transmission including a pair of differential-gear sets in respective branches thereof, these differential-gear sets having codirectionally rotatable driving inputs and oppositely rotatable steering inputs. Differential rotation can be imparted to these steering inputs from the engine via a continuously adjustable hydraulic power train and/or a supplemental mechanical power train through the intermediary of a pair of cascaded planetary-gear sets, the hydraulic power train comprising a pair of hydrostatic motors fluidically linked with an engine-driven pump or pair of pumps and drivingly coupled with respective inputs of the first-stage planetary-gear set. When driving along a road, the operator of such a vehicle uses the hydraulic power train for precise maneuvering; in the field, however, the mechanical power train enables sharper turns to be carried out.
If a hydraulic torque converter is included in the transmission between the engine and the driving inputs of the differential-gear sets associated with the traction elements, problems may arise in situations of negative load-- as during downhill driving--when the turbine on the output side of the converter outruns its impeller on the input side. As the engine slows down, the torque transmitted by the mechanical power train and added to that of the hydraulic train may not suffice to steer the vehicle through sharp curves along a descending road. While the steering torque could be intensified by accelerating the engine, the driver will instinctively act in the opposite way by further throttling the fuel supply.